Reflected Light Bireflectance

Reflected light microscopy is used to examine opaque
minerals (and other materials, e.g.. ceramics) to determine the paragenetic
relationships between different mineral phases and their identification. Often,
the same specimen which is viewed using the light microscope can be analyzed
using advanced x-ray and ion microprobe techniques.

The sample (polished thin section, epoxy grain mount, or polished section) is
placed in the appropriate reflected light microscope. Bireflectance is an
optical effect similar to pleochroism where the mineral appears to change in
intensity as it is rotated while illuminated by plane polarized light. The
polarizers are not crossed to observe bireflectance.

Isotropic minerals (eg, galena, pyrite) do not show any bireflectance (or
pleochroism) when rotated in plane polarized light. Minerals which are
pleochroic are also bireflectant. Care must be taken when observing
bireflectance to follow these rules: